Zeolites typically are crystalline hydrated aluminosilicates of Group I and Group II elements. In particular, those elements include sodium, potassium, calcium, magnesium, barium and strontium. The structure of zeolite is typically an aluminosilicate framework based on an indefinitely extending three dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing of the oxygens. Zeolites are often represented by the empirical formula M.sub.2/n O.Al.sub.2 O.sub.3. x SiO.sub.2. y H.sub.2 O. In this oxide formula, "x" is generally greater or equal to 2 since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and "n" is the Group I or Group II cation valence. The framework contains channels and interconnected voids which may be occupied by the cation and water molecules. The cations are often quite mobile and may be exchanged by other cations. Intracrystalline zeolitic water may be reversibly removed. In some zeolites, cation exchange or dehydration may produce structural changes in the framework. It is known that gallium ions and silicon ions may to some limited extent be substituted for aluminum cations in the framework. Germanium has similarly reportedly been substituted into the zeolitic framework.
Much zeolite research has been focused on the synthesis of zeolite frameworks containing elements other than silicon and aluminum. While an extensive family of aluminum-phosphorus zeolites has recently been synthesized, the substitution of other elements is the subject of major controversy in the zeolite literature. For instance, U.S. Pat. Nos. 3,329,480 and 3,329,481 both issued to D. A. Young, report the existence of crystalline zirconosilicate and titanosilicate zeolites. A zeolite having chromium in the tetrahedral positions has been described by Yermolenko et al at the Second Oil Union Conference on Zeolites, Leningrad, 1964, pages 171-8 (published 1965). However, D. W. Breck in Zeolite Molecular Sieves, p. 322, John Wiley & Sons (1974) suggests that the chromium present was not present in a zeolite A structure and furthermore was present as an impurity in insoluble form. The impurity was said to be in the form of a chromium silicate as confirmed by the nature of the water vapor adsorption isotherm. The overall status of tetrahedral substitution has recently been reviewed by Barrer in "Hydrothermal Chemistry of Zeolites", Academic Press (1982), Chapter 6.
Because of the presence of phosphorus in tetrahedral PO.sub.4 units in certain rare zeolites, extensive work has been done to synthesize zeolites containing PO.sub.4 tetrahedra. Various phosphorus containing zeolites have been prepared as reported in Breck, supra, p. 323 et seq. The synthesis technique for production of phosphorus-containing zeolites generally involves crystallization from a gel in which the phosphorus is first incorporated by a controlled copolymerization in co-precipitation of all of the component oxides in the framework, i.e., aluminate, silicate, and phosphate in the homogeneous gel phase. The crystallization of the gel is then carried out at a temperature between 80 and 210.degree. C.
The chemical treatment of zeolites with silicon tetrachloride and tetrafluoride has been demonstrated as a very successful method of replacing Al.sup.3+ with Si.sup.4+ in zeolite frameworks. Major literature has developed around this phenomenon since it was first reported by Beyer and Belenykaja, ("Catalysis by Zeolites" No. 5, Editor B. Imelik et al, (1980), Elsevere Press). Experiments with tin tetrachloride reported in the same volume by Otsuka et al produce no evidence for inclusion of the tin into the zeolitic framework and reported only low temperature adsorption of small amounts of tin tetrachloride and the resultant effect of that compound on catalytic activity. Otsuka's experiments were aimed at evaluating the particular zeolite as a support for the strong Lewis acid tin tetrachloride.
Inclusion of tin into zeolites for one reason or another is known. For instance, U.S. Pat. No. 3,013,987 to Castor et al, issued Dec. 19, 1961, suggests the adsorption of various elemental transition metals, including tin, into such large pore zeolites as natural faujasite and synthetic zeolites X, Y, and L. The process is accomplished by either adsorbing organometallic species with the dehydrated zeolite or by solution adsorption of similar soluble species. Tin, an element of Group IV A, is preferably reacted as a tin alkyl. After introduction of the metal-containing compound to the zeolite, the compound is broken down by, e.g., reduction with hydrogen, to effect the deposition of elemental metal.
U.S. Pat. No. 3,200,082 to Breck et al, issued Aug. 10, 1965, has the similar objective of occluding reduced metal in the zeolite voids. Therein, it is suggested that the incorporation of metal into the particular zeolitic molecular sieves is limited by the extent to which the molecular sieves may be ionexchanged with the desired cations. Consequently, both Castor et al and Breck et al are considered to teach only processes and compositions in which the included tin is found in the sorption or ion-exchangeable sites and not in the tetrahedral framework itself.
U.S. Pat. No. 3,600,301 to Rausch, issued Aug. 17, 1971, discloses a catalytic composite material used in hydrocarbon hydroprocessing. Tin and a Group VIII noble metal component are introduced into a porous carrier material, preferably crystalline zeolite. The various methods include coprecipitation or co-gellation of the tin containing material with a carrier material. A further method includes impregnation of the carrier zeolite with a tin-containing compound. Regardless of the manner by which the tin and other components are added to the carrier material, the final composite generally is said to be dried at a temperature of about 200.degree. F. to about 600.degree. F. for a period of 2 to about 24 hours or more and finally calcined at a temperature of about 700.degree. F. to about 1100.degree. F. for a period less than about 10 hours in order to convert the metallic components contained therein to the oxide form. The objective is said to be to deposit the metal within the void volume of the zeolite or on its surface.
None of this literature appears to disclose either a zeolite which contains tin in its framework structure or suggests any processes for attaining such composition.